Biogas is a sustainable alternative source of energy, for example can be used as a low cost fuel, but to date there is still a lack of efficiency in its production.
Biogas is produced from the anaerobic digestion of organic matter such as animal manure, sewage, and municipal solid waste. The process produces methane and carbon dioxide. After the biogas is processed to required standards of purity, biogas becomes a renewable substitute for natural gas and can be used to fuel natural gas vehicles, helping to replace fossil fuels.
Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen, used for industrial or domestic purposes to manage waste and/or to release energy. It is widely used as part of the process to treat wastewater. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is widely used as a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production, helping to replace fossil fuels. The nutrient-rich digested biomass which is also produced can be used as fertiliser. The technical expertise required to maintain industrial scale anaerobic digesters coupled with high capital costs and low process efficiencies had limited the level of its industrial application as a waste treatment technology.
Many attempts have been performed in order to increase biogas production, including thermal or ultrasound pre-treatment of organic waste, but all of them present limited industrial applications.
Additionally, iron has been shown to enhance anaerobic digestion, but there are severe drawbacks for introducing the metal ion in an anaerobic closed reactor. For example, the addition of large iron oxide macroscopic crystal lacks concentration control, thus, either the ions iron concentration is too high at the beginning or it reaches too low concentration too rapidly.
US patent application U.S. Pat. No. 4,981,593 describes a process for removing of suspended impurities from residual waters. This document describes that carrying out an anaerobic digestion with magnetite particles of size about 50-100 mm allows removing the H2S from the reactor environment. It also describes the strong tendency for sewage organics to co-flocculate with coagulant/adsorbents such as magnetite, a ferromagnetic mineral with chemical formula Fe3O4 and that coagulant/adsorbent particles should have a particle size of 2 10−5 m or less, preferably 10−6 to 10−5 m. However, the results of digester operation with the organic material still attached to the magnetite particles of size about to 10−6-10−5 m, were disappointing. The gas production was an order of magnitude below what would be expected from a digester. As a result, the document concludes that improved digestion can be achieved by stripping and separating the concentrated sewage material from the magnetite particles and then feeding the resultant slurry to an anaerobic digester. In this document it is also described that the anaerobic digester may contain magnetite particles of size about 50-100 mm. This means that this patent publication describes that when the particle size of magnetite increase the gas production increase too.
Thus, from what is known in the art, it is derived that the development of a process for the production of biogas is still of great interest.